Below are two statements from my notes and I am trying to verify them explicitly. In both cases the fields are assumed to transform under the fundamental representation of $O(N)$ -

--'The kinetic term for a Dirac spinor is invariant under the symmetry group $U(N) \otimes U(N)$'.

I considered the case of a Weyl spinor first. This has a kinetic term $i \bar \phi_R \gamma^{\mu} \partial_{\mu} \phi_R$ and if $\phi_R \rightarrow U \phi_R$ then $i \bar \phi_R \gamma^{\mu} \partial_{\mu} \phi_R \rightarrow i\phi^{\dagger}_R U^{\dagger} \gamma_0 \gamma^{\mu} \partial_{\mu} U \phi $. Because $U$ and the gamma matrices act on different spaces, can I just shift the $U$ to the $U^{\dagger}$ and then using $UU^{\dagger}=1$ get the result? The $U(N) \otimes U(N)$ for the Dirac spinors comes about from decomposing a Dirac spinor into its left and right handed components each of which transforms under a 'left handed fundamental representation' or 'right handed fundamental representation' so could write the symmetry group as $U_L(N) \otimes U_R(N)$ (correct if mistaken).

--'If $T_a$ are the generators of $O(N)$, the bilinears $\phi^T T^a \phi$ transform according to the adjoint representation.'

I'm just wondering do generators always transform in the adjoint representation themselves? I read on here in another thread that the adjoint representation can be thought of as the representation anchored at the identity so if anyone could shed some light on this statement that'd be great.


2 Answers 2


Yes, the first part of your question is appreciated and answered soundly. The fermion kinetic term splits into two independent parts involving left and right Weyl spinors respectively, so each is independent under a separate U(N) as your wrote down.

The second question is a matter of language. A generator is a matrix with one adjoint index, a here, ranging over the dimension of the Lie algebra, so "universal" as per your question; and two indices each corresponding to its representation, e.g. i,j, the matrix indices, ranging over the dimension of that particular representation. It is an operator of the representation, acting on vectors of it, such as your φ. If your φ is in the fundamental, for instance, T will act on it as $\phi_i\mapsto T^a_{ij} \phi_j$, all in the fundamental. Dotting this with another vector, φ will yield a scalar in the fundamental, your expression, with a loose index a of the adjoint, so, then a vector in the adjoint. Always in the adjoint, regardless of what irrep φ you started with, as long as you used the suitable representation matrices for that generator.

To rotate this vector under O(N), you'll have to act on it with the Lie algebra structure constants, which are the operators T in the adjoint, so, analogously, $\phi^T T^a \phi\mapsto i f_{abc}~\phi^T T^c \phi$. For instance, for O(3) the generators are the familiar vector spin matrices $i\epsilon_{abc}$s.

  • $\begingroup$ Thanks for your answer! So do the generators always transform under the structure constants? - i.e do generators always transform under the adjoint representation? I've seen the equation $$u^{\dagger} T_i u = R_{ij} T_j$$ where $u$ is an element of a group come up a lot. $\endgroup$
    – CAF
    Apr 10, 2016 at 19:58
  • $\begingroup$ Well, if you wish to put it that way: The label of the generator always transforms in the adjoint, by the structure constants. In the relation you wrote, in any rep, you are looking at a similarity transform of a generator, where the us are exponentials of generators, exp(ia.T). The action of these exponentials on T is in fact adjoint, so your expression is Tj-i[a.T,Tj] +... so then a rotation of the J indices, only. Here, the j's are adjoint, and only they transform--the other two irrep indices just serve in the matrix multiplications. $\endgroup$ Apr 10, 2016 at 21:20
  • $\begingroup$ You switched indices and I fell for it, so as to maximally ambiguate them. In proper notation consistent to your question and my answer, my above reply reads: $u^\dagger T_a u= e^{-i \theta\cdot T} T_a e^{i\theta\cdot T}= e^{-i{\mathrm ad} \theta\cdot T} T_a= T_a -i [\theta\cdot T, T_a]+...=R^\theta _{ab} T_b$ . $\endgroup$ Apr 10, 2016 at 21:42
  • $\begingroup$ Thanks! My last question is when would I use the transformation law $T_a' \rightarrow u^{\dagger}T_a u$ for generators? Like for example, in $SU(2)$ the Pauli matrices comprising the fundamental representation do not transform. $\endgroup$
    – CAF
    Apr 11, 2016 at 20:53
  • $\begingroup$ This one I didn't understand: In the eqn right above your question, true for absolutely all representations, halved Pauli matrices do just that: $T_a=\sigma^a/2$, and using the explicit representation of the exponential of them you find the exact rotation formula for $\sigma^a$. $\endgroup$ Apr 11, 2016 at 21:27

(I) Assuming there are $N$ distinct fermions in your Lagrangian (e.g. quarks in the standard model), the kinetic term will have the $U(N)\times U(N)$ symmetry. This symmetry is broken by the mass term, however, which couples fermions with different handedness.

(II) If $V$ is the fundamental representation of a Lie group (with dual/conjugate representation $V^*$), generators can be identified as a subspace of $V\otimes V^*$. If $g$ is an element of the Lie group, then $g$ 'acts on' $V\otimes V^*$ as follows: $g\cdot (v\otimes \omega)=(U(g)v)\otimes (U(g)' \omega)=U(g)v\otimes \omega U(g^{-1})$. (Often it's possible to choose a unitary representation, so that $U(g^{-1})=U(g)^\dagger$).

[Note that the tensor representation $V\otimes V^*$ is reducible, and only contains the adjoint representation as a subrepresentation].

Now it turns out that a large class of Lie groups can be expressed locally as the exponential of some element in the tangent space near the identity: i.e. $$ U(g)=\exp(-iA_a(g)\tau^a), $$ where $A_a(g)$ are some $g$ dependent coefficients (local coordinates for the Lie group), and $\tau^a$ are the generators. The exponential is interpreted as a power series, when sensible (when $V$ is infinite dimensional, as in quantum mechanics, it's typical to specify that the spectrum of $A_a\tau^a$ is bounded from below, so that analytic continuation can be used).

These local coordinates for the Lie group imply that the group action defined above induces an action of the tangent space near the identity (i.e. the Lie algebra associated with the group): $\mathcal A \cdot v\otimes \omega=[\mathcal A, M_{v\otimes\omega}]$, where $M_{v\otimes\omega}$ is the 'matrix' associated with $v\otimes \omega$.

('Trivial' (in the common sense) addendum: the reason higher order terms in the power series expansion of $U(g)$ do not give alternative adjoint representations of the Lie algebra is essentially a kind of 'memory effect', where the linear term interferes with higher order components).


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